The present invention relates to the growth of cubic silicon carbide crystals. More specifically, the present invention relates to the growth of cubic silicon carbide by Chemical Vapor Deposition (CVD).
A great need has developed to produce high-temperature electronic devices for advanced turbine engines, geothermal wells, and other applications. Cubic silicon carbide (SiC) is considered an excellent candidate for use as a high temperature electronic material because of its large band gap, good carrier mobility and excellent physical stability. A primary reason, though, why development of cubic SiC has not been carried further is the lack of a reproducible process for producing the single crystal substrates necessary for device fabrication.
A method that attempts to remedy this problem is presented by S. Nishino, J. A. Powell, and H. A. Will, "Production of large-area single-crystal wafers of cubic SiC for semiconductor devices," Appl. Phys. Lett. Vol. 42, No. 5, Pages 460-462, March 1983. However, the method of Nishino et al. results in cubic SiC crystals that are not of a high purity and have poor duplicability. (See A. Addamiano and J. A. Sprague, "Buffer-layer technique for the growth of single crystal SiC on Si," Appl. Phys. Lett. Vol. 44, No. 5, March 1984).
There are several reasons why the method of Nishino et al. is not adequate. Nishino et al. teach etching of the Si substrates, on which the cubic SiC crystals are grown in the apparatus used for the CVD, using hydrogen chloride (HCl) vapor at a temperature of about 1200.degree. C. Etching of the substrates in the CVD reactor with hot HCl vapor results in considerable erosion of the graphite susceptor housing the Si substrates. This, in turn, results in random deposition of carbon particles on the Si substrates, with the consequence that there is poor duplicability of the results and impurities are introduced into the cubic SiC as it is grown. Another reason the method of Nishino et al. is not adequate is because in the formation of a buffer layer of cubic SiC on the Si substrate Nishino et al. teach mixing H.sub.2 and C.sub.3 H.sub.8 at room temperature, flowing the mixture over the Si substrates at room temperature, then bringing the temperature to 1360.degree. C. and maintaining the temperature for about 1 minute, followed by cooling. The buffer layers formed by this technique are stated to be mainly polycrystalline cubic SiC, by Nishino et al., supra. This is because C.sub.3 H.sub.8 is present throughout the heating step causing different growth conditions for cubic SiC that forms as the temperature is raised from room temperature to 1360.degree. C.
Nishino et al. also teach to carry out CVD on the buffer layers at a temperature of about 1360.degree. C. This temperature is not the ideal temperature for formation of perfect cubic single crystal layers.